“Data show that middle-income households have continued to move down, and less so up, the income distribution in the United States since the 1970s—a phenomenon that is often referred to as the polarization or “hollowing out” of the income distribution. While the level of income polarization is generally lower in the richer states (defined as those with higher median household income levels), there have been wide variations in income polarization over time and across states. We develop two indices to measure income polarization, including a novel hollowing-out index. We also examine the proximate causes of income polarization, using an econometric analysis at both state and household levels. The results suggest that technology, measured by job routinization, and international trade, measured by job offshoring, can explain more than half of the rise in income polarization, with broadly equal contributions. Household characteristics, particularly better education, have had important countervailing effects on income polarization. Policies should continue promoting technological progress and international trade, but also recognize that these positive forces have important side-effects on the income distribution and household welfare that need to be understood and mitigated.”